Metals and carbon materials have conventionally been used for purposes requiring high conductivity. Carbon materials, in particular, being materials free of the corrosion that is characteristic of metals and exhibiting excellent heat resistance, lubricating properties, thermoconductivity and durability, have come to play an important role in fields such as electronics, electrochemistry, energy, transport vehicles and the like. Composite materials made of combinations of carbon materials and polymer materials have also reached dramatic levels of development, and have come to serve in a capacity of ever increasing performance and functionality. Greater freedom in shaping processability permitted by combinations with polymer materials in particular has been one reason for the increased development of carbon materials in fields requiring conductivity.
In recent years, in light of problems related to the environment and energy, greater attention has been focused on clean electric generators wherein fuel cells generate electricity by the reverse reaction of water electrolysis, using hydrogen and oxygen, and producing no waste other than water, and carbon and polymer materials also play an important role for these as well. Solid polymer fuel cells appear to be most promising for use in automobiles and people's use when the purpose is operation at low temperatures. Such fuel cells can exhibit high-output electricity generation by lamination of simple cells composed of a polymer solid electrolyte, gas diffusing electrodes, a catalyst and a separator.
The separators used to partition the simple cells usually have grooves for supply of fuel gas and oxidizing gas, and must have high gas impermeability to achieve complete separation of the gases, as well as high conductivity to reduce internal resistance. Other requirements include thermal conductivity, durability and strength.
In order to satisfy these requirements, such separators have traditionally been examined from the standpoint of both metals and carbon materials. The corrosion problem associated with metals has invited attempts to coat the surfaces with precious metals or carbon, but it has not been possible to achieve adequate durability, and the costs of coating have also been a problem.
On the other hand, carbon materials have also been extensively studied, examples of which include molded articles obtained by press molding expanded graphite sheets, molded articles obtained by impregnating a sintered carbon material with a resin and curing, and molded articles obtained by mixing and molding thermosetting resin-fired glassy carbon, carbon powder and a resin.
For example, in Japanese Unexamined Patent Publication (Kokai) HEI No. 8-222241 there is disclosed a complex process in which a bonding material is added to carbonaceous powder to ensure reliability and dimensional precision and the mixture is heated and formed by CIP, after which this is fired and graphitized to produce an isotropic graphite material which is then impregnated with a thermosetting resin, subjected to curing treatment, and carved with grooves by a cutting step. Also, Japanese Unexamined Patent Publication SHO No. 60-161144 discloses impregnating a thermosetting resin into paper comprising carbon powder or carbon fiber, and then laminating the paper, pressing the laminate and firing it. Japanese Unexamined Patent Publication No. 2001-68128 discloses injection molding of a phenol resin into a separator-shaped die and firing it. Such fired materials exhibit high electroconductivity, but the long time required for firing lowers the productivity. When a cutting step is necessary, the productivity is further lowered and costs are increased, such that numerous problems have prevented their future use as common materials.
On the other hand, molding methods have been considered as means of increasing productivity and lowering costs, and the materials used therefor have generally been composites of carbon-based fillers and resins. For example, Japanese Unexamined Patent Publication SHO No. 58-53167, Japanese Unexamined Patent Publication SHO No. 60-37670, Japanese Unexamined Patent Publication SHO No. 60-246568, Japanese Examined Patent Publication (Kokoku) SHO No. 64-340 and Japanese Examined Patent Publication HEI No. 6-22136 each disclose a separator made of a thermosetting resin such as a phenol resin, graphite or carbon, Japanese Examined Patent Publication SHO No. 57-42157 discloses a bipolar separator made of a thermosetting resin such as an epoxy resin and an electroconductive material such as graphite, and Japanese Unexamined Patent Publication HEI No. 1-311570 discloses a separator comprising a mixture of expanded graphite and carbon black with a thermosetting resin such as a phenol resin or furan resin.
Separators employing composites of a carbon-based filler and a resin require a drastically greater content of the carbon-based filler to exhibit high conductivity, but because a large amount of resin must be added to maintain moldability, it has been difficult to achieve sufficiently high conductivity. It has also been attempted to shape the composition into a highly precise sheet by rolling or the like and harden it into a fuel cell separator shape, for the purpose of improving thickness precision which is a particularly important characteristic for fuel cell separators, but since the resulting sheet is not uniform when it contains a high content of carbon-based filler, it has been necessary to increase the amount of resin serving as the matrix. As a result, it has not been possible to achieve adequate electrical conductivity and thermal conductivity.
When, for higher electrical conductivity, a firing step is included wherein the molded article is heated for a long period at a high temperature of 1000-3000° C., the length of time required for production is extended and the production process becomes complicated, thereby further increasing the cost.
It is an object of the present invention, which has been accomplished in light of these circumstances, to provide a conductive curable resin composition with loadability of a high content of conductive filler and with excellent shaping processability. It is another object thereof to provide a fuel cell separator an assembly for a cell, an electrode or a heat releasing plate with excellent conductivity and heat release properties, which is obtained by shaping the above-mentioned composition into a sheet while it is still uncured, and curing it into a fuel cell separator shape an assembly for a cell, an electrode or a heat releasing plate, as well as a process for their production.